Hi,
Really liking Choo so far! One thing I was missing was the ability to define an async route (I'm working on a game and wanted to preload assets). I didn't see a simple way to accomplish this (maybe I missed something obvious), so I wrote a little module that augments app with an `asyncRoute` method:

```javascript
const app = withAsyncRoute(choo());

app.asyncRoute('/', loadResources,
() => html`

`,
(state, emit, data) => html``
(state, emit, err, params) => {
console.error('error loading /:', params, err, state);
return html`

`;
},
);
```

Was wondering if there's interest in either merging this function into Choo proper, or as a third-party npm module?

For reference, here's the module -- it's pretty simple:

```javascript
module.exports = (app) => Object.assign(app, {
/**
* @param {string} route
* @param {Promise|Function>} promise
* @param {Function} loadingHandler
* @param {Function} loadedHandler
* @param {Function} errorHandler
*/
asyncRoute: (route, promise, loadingHandler, loadedHandler, errorHandler) => {
app.use((state, emitter) => {
const emit = emitter.emit.bind(emitter);

app.router.on(route, (params) => {
state.params = params;

if (typeof promise === 'function') {
promise = promise();
}

let completed = false;
let isError = false;
let data = null;

promise.then(result => {
completed = true;
data = result;
emitter.emit('render');
}).catch(err => {
completed = isError = true;
data = err;
emitter.emit('render');
});

return () => {
if (!completed) {
return loadingHandler(state, emit, params);
} else if (isError) {
return errorHandler(state, emit, data, params);
} else {
return loadedHandler(state, emit, data, params);
}
};
});
});
}
});
```

Review the existing app.use and app.router.on APIs before assessing how asyncRoute would fit Choo. Compare the proposed loading, loaded, and error handlers with current route behavior and determine whether the feature belongs in Choo or as a third-party module; done requires an agreed scope and implementation direction.

Written by the indexing model from the issue text.